The present invention relates to nuclear fusion reactors, and, more particularly, to an apparatus and method of enhancing the toroidal plasma current and the fuel ion reactivity of a fusion reactor by injecting waves into a toroidal plasma to channel energy from energetic ions directly to electrons and fuel ions.
To achieve ignition or efficient burnup of fuel ions (deuterium and tritium) in a magnetic confinement fusion reactor, a dense plasma must be confined and heated to extremely high temperatures. In a tokamak-type fusion reactor, plasma confinement is obtained by a toroidal magnetic field supplemented by a poloidal magnetic field produced by a toroidal electric current in the plasma itself. This current also serves to heat the plasma by resistive heating (I.sup.2 R). For a tokamak reactor to operate economically, the toroidal plasma current must be induced efficiently and in long pulses or steady state modes.
Currently, the toroidal plasma current is induced primarily by means of a time varying magnetic field produced by large external transformers. This method, however, suffers in that it is inherently a pulsed method. Other methods of driving the plasma current, as described in U.S. Pat. Nos. 4,425,295 (Fisch et al.) and 4,292,124 and 4,423,001 (Fisch), utilize radio-frequency (rf) waves to selectively heat electrons or minority ions to generate the toroidal current. These methods, though able to produce steady-state plasma current, nonetheless, require the addition of large amounts of external energy.
An additional problem associated with the tokamak is its ability to sufficiently heat the deuterium and tritium fuel ions to achieve substantial fusion reactivity. It is apparent that additional increases in the fusion reactivity will be necessary to obtain an economically feasible tokamak reactor, particularly when the plasma fuel comprises deuterium alone or deuterium and .sup.3 He. Some of the methods used to increase the fusion reactivity are neutral beam injection, induction of plasma compression and turbulence, and injection of waves at various frequencies and phases to heat the ions. Although some of these methods have been more effective than others, all have the same shortcoming in that they require the addition of large amounts of external energy into the system.
What is needed is a mechanism whereby some of the energy released from nuclear fusion can be converted, in situ, directly to the electrons and fuel ions of the plasma so that the toroidal plasma current may be generated efficiently and continuously and the fusion reactivity of fuel ions increased. The advantage of an in situ energy conversion is that it avoids the inherent inefficiencies associated with the process of extracting the energy released from fusion and then using the extracted energy to generate and deliver useful power back into the plasma. Therefore, even if only a small fraction of the fusion energy can be tapped in situ to drive the toroidal plasma current and enhance fusion reactivity, because of the several-fold energy savings over recirculated power, the savings could mean the difference between a marginal or economically feasible reactor.
An effective means for delivering continuous power to a tokamak reactor has been through the introduction of various waves, such as lower hybrid waves, into the plasma. Lower hybrid waves are a demonstrated, continuous means for driving the plasma current as well as for increasing the reactivity of fuel ions. For instance, when lower hybrid waves are absorbed by electrons traveling in one direction in the toroidal plasma, they gain energy thus enhancing the plasma current. Likewise, when lower hybrid waves are absorbed by the fuel ions, their energies are increased to enhance fusion reactivity.
Unfortunately, it has been thought that in most wave regimes operating in a tokamak reactor, the power delivered to the plasma by the waves becomes diminished because the same waves are also absorbed by energetic ions which are produced by nuclear fusion. It has been thought that energetic ions tend to absorb more wave power than electrons, in part, because the electron velocity distribution is flat in the regions where the wave phase velocities are finite. On the other hand, the energetic ion interaction exhibits no saturation, because, once in resonance, energetic ions remain in resonance even as they gain energy. However, the calculations which explore this severe damping by energetic ions, were made under the assumption that the plasma is infinite and homogeneous. We show, in this invention, that in an inhomogeneous plasma, subjected to properly localized and phased wave power, the energetic ions, instead of damping the wave energy, can actually amplify the wave. The amplification is then transferred directly to the plasma, via wave damping by electrons or fuel ions, to enhance respectively the plasma current and fuel ion reactivity.
In view of the foregoing, the general object of this invention is to provide an apparatus and method of extracting energy from fusion by-products, or energetic ions, and channeling the extracted energy back into the plasma by injecting waves of predetermined frequency and phase into a toroidal plasma.
Another object of this invention is to provide an apparatus and method of injecting waves into a toroidal plasma to channel energy from energetic ions directly to electrons to generate and enhance the toroidal plasma current of a fusion reactor.
Yet another object of this invention is to provide an apparatus and method of injecting waves into a toroidal plasma to channel energy from energetic ions directly to fuel ions to enhance the fusion reactivity of a fusion reactor.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and practice of the invention.